This invention relates in general to overlay error detection and, in particular, to an imaging system for detecting misalignment of two structures and methods for determining overlay errors.
Overlay error measurement requires specially designed targets to be strategically placed at various locations, normally in the street area between dies, on the wafers for each process. The alignment of the two overlay targets from two consecutive processes is measured for a number of locations on the wafer and the overlay error map across the wafer is analyzed to provide misalignment information. This information may then be used for the alignment control of wafer processing equipment.
A key process control parameter in the manufacturing of integrated circuits is the measurement of overlay target alignment between successive layers on a semiconductor wafer. If the two overlay targets are misaligned relative to each other, the electronic devices fabricated will malfunction and the semiconductor wafer will need to be reworked or discarded.
Typically, conventional overlay targets are box-in-box targets and bar-in-bar targets. A box-in-box target typically has a 10 μm inner box and a 20 μm outer box. The outer box is printed on the substrate (or previous process layer) and the inner box is resist printed on the current layer. Overlay error is reported as the mis-position of the inner box with respect to the outer box. A bar-in-bar target also has a 10 μm inner target on the current layers and a 20 μm outer target on the previous layers. However, the box edge is replaced with a narrow bar approximately 2 μm wide. The box-in-box targets are more compact; however, the bar-in-bar targets provide better measurement performance. Overlay targets may comprise grating structures on top of the wafer or etched into the surface of the wafer. For example, one overlay target may be formed by etching into the wafer while another adjacent overlay target may be a photoresist layer at a higher elevation over the wafer.
Conventional systems for detecting overlay target misalignment typically employ an electronic camera that images the “box-in-box” or “bar-in-bar” target. The accuracy of the conventional system is limited by the accuracy of the line profiles in the target, by aberrations in the illumination and imaging optics and by the image sampling in the camera. Such methods are complex and they require full imaging optics. Vibration isolation is also required and it may be difficult to integrate such systems into process equipment, such as a track. Conventional methods for overlay error detection have difficulties with low contrast targets, such as those obtained by chemical and mechanical polishing processes.
An improvement to the conventional method is described in U.S. Pat. No. 6,023,338. This patent discloses a method where two overlay target structures are placed next to each other and two radiation beams illuminating spots on the structures are scanned along two separate paths across portions of both structures. The intensity of the radiation reflected along both paths are detected and processed to calculate any offset between the two structures.
None of the above-described methods and techniques are entirely satisfactory. It is, therefore, desirable to develop an improved system with better performance and simplified characteristics.